Ensure your data (at least all Home folders) is backed up before you install. Hard disks can fail at any time so it is important to regularly backup to an external disk with Time Machine or a third party alternative. Inexperienced users should follow Apple's install advice but experienced users may prefer to do a clean install.
Those wishing to upgrade to Leopard should be aware that install disks can be expensive. Details: http://lowendmac.com/deals/best-os-x-leopard-prices.html Standard Leopard installers impose several hardware limitations including speed and RAM size but all these restrictions can be overcome. Google for details. Leopard works well at 500 MHz with 1 GB of RAM and many happy users have less than this.
Upgrade beyond Leopard
OSs beyond OS X 10.5.8 require an Intel processor. If in doubt click the apple at the top left of your screen and select 'About this Mac'. This will give you your OS number. Then click 'More Info' to see which processor you have. If it says PowerPC you cannot upgrade to Snow Leopard and above. If you have an Intel Mac it is well worth upgrading to Snow Leopard now and then considering other options after that. You can buy Snow Leopard here: http://store.apple.com/us/product/MC573Z/A